mm: fix null pointer dereference in wait_iff_congested()

mm: fix null pointer dereference in wait_iff_congested()

An unintended consequence of commit 4ae0a48b5efc ("mm: modify
pgdat_balanced() so that it also handles order-0") is that
wait_iff_congested() can now be called with NULL 'struct zone *'
producing kernel oops like this:

  BUG: unable to handle kernel NULL pointer dereference
  IP: [<ffffffff811542d9>] wait_iff_congested+0x59/0x140

This trivial patch fixes it.

Reported-by: Zhouping Liu <[email protected]>
Reported-and-tested-by: Sedat Dilek <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Zlatko Calusic <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>